Once cancer is diagnosed, it is hard to tell what caused it, Claiborne said. Indicting mammography can frighten women away from a test able to detect tumors too small to be felt, she said.

The American Cancer Society recommends an initial screening at age 35. From age 40 to 49, tests are urged every one to two years, and every year beginning at 50.

Screening mammography involves taking low-dose X-rays of the breast. The pictures can reveal micro-calcifications - the beginnings of a tumor - two to three millimeters in diameter, about the size of the period at the end of this sentence.

To obtain the best views of the tissue and its ducts, the breast is compressed on the machine's platform. The Canadian National Breast Cancer Screening Study, which involved 90,000 women, found that the compression may accelerate cancer.

Dr. Linda Persohn, a diagnostic radiologist at Florida Hospital's Center for Women's Medicine, said she doubts squeezing a breast on a platform would cause cancerous cells to flow into blood vessels.

The magazine Diagnostic Imaging, which printed the preliminary study results, also reported that the Canadian study had found that women who underwent the screenings were 50 percent more likely to die.

''This number is so far out of the realm of being conceivable that it makes you wonder if they've skewed their own data,'' Persohn said. She said she believes the Canadian research is hopelessly flawed.

The Canadian study had been the subject of speculation for months as details leaked about a surprisingly high death rate among women under 50 who got mammograms.

An adviser to the study, Dr. Sam Shapiro of Johns Hopkins University, confirmed that researchers had found when younger women underwent the screenings they were more likely to die of breast cancer than those getting physical exams alone. But he said the increase could have been a statistical fluke.

Nancy Christiansen, a nurse who specializes in breast imaging at the Center for Women's Medicine said she uses a simple rule of thumb when considering the potential cancer risk from mammography.

''The risk of one mammogram equals the cancer risk of smoking three-fourths of a cigarette over a lifetime,'' Christiansen said, citing a comparison from one study.